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Hi all,
I'm new to the nanovna and VNAs in general.
I got a nanoVNA-H4 recently and was playing with measuring a few antennas
(SWR and reactance) to find my way around.
I noticed that the measurement was depending very much on where the VNA was
located, and where my hands were located relative to the VNA.
I expected that to be true for the antennas themselves, i.e. when I touch
the antenna or come close to it, measurements will change.
But the measurements were e.g. changing extremely when I put my hand on the
side of the nanovna opposite from the SMA connectors.
Is that expected behaviour? Is there some form of shielding in the
enclosure?
I got the device from a reputable dealer in the US who was claiming these
are "validated to be good production models" (whatever that means).

Any hints?

Hello,

Yes, unless you prevent the return current of the antenna from passing to the vna, the vna is part of the antenna system. A choke, a balun, or an isolator is required to decouple the measuring device from the device under test.

Alan

There are several message threads about this. Since you have not grounded the Nanovna, you are part of the antenna system and where and how you hold the device will change its readings.?
Search for: antenna near field
/g/nanovna-users/message/5796?p=,,,100,0,0,0::Created,,+field+antenna,100,2,0,37711267



On Sat, 22 Aug 2020 at 8:10 AM, C Gosch via groups.io<ch.gosch@...> wrote: Hi all,
I'm new to the nanovna and VNAs in general.
I got a nanoVNA-H4 recently and was playing with measuring a few antennas
(SWR and reactance) to find my way around.
I noticed that the measurement was depending very much on where the VNA was
located, and where my hands were located relative to the VNA.
I expected that to be true for the antennas themselves, i.e. when I touch
the antenna or come close to it, measurements will change.
But the measurements were e.g. changing extremely when I put my hand on the
side of the nanovna opposite from the SMA connectors.
Is that expected behaviour? Is there some form of shielding in the
enclosure?
I got the device from a reputable dealer in the US who was claiming these
are "validated to be good production models"? (whatever that means).

Any hints?

On 8/21/20 10:59 PM, C Gosch via groups.io wrote:

Hi all,
I'm new to the nanovna and VNAs in general.
I got a nanoVNA-H4 recently and was playing with measuring a few antennas
(SWR and reactance) to find my way around.
I noticed that the measurement was depending very much on where the VNA was
located, and where my hands were located relative to the VNA.
I expected that to be true for the antennas themselves, i.e. when I touch
the antenna or come close to it, measurements will change.
But the measurements were e.g. changing extremely when I put my hand on the
side of the nanovna opposite from the SMA connectors.
Is that expected behaviour? Is there some form of shielding in the
enclosure?
I got the device from a reputable dealer in the US who was claiming these
are "validated to be good production models" (whatever that means).

I'm assuming small changes, not huge gyrations.

totally to be expected - What frequency were you working at? How long is the coax from the antenna and how far are you from the antenna.

For virtually all antennas, the coax (and anything connected to it) becomes part of the antenna, unless you've been very careful with chokes and layout.

let's say you're measuring something like a FM broadcast band (88-108 MHz) antenna and it's sitting on a stand - how does the feedline get to the antenna? Is it perfectly perpendicular and symmetrically placed? Probably not - so some of the fields from the antenna couple to the feedline, travel along the outside of the feedline to the VNA. And you.

Are you standing/sitting within a few wavelengths of the antenna? In any direction, not just the "beam" direction if any - You're in the near field. Is the NanoVNA within that distance? It's in the near field.


If you're getting large changes in apparent |S11|, especially when you move it - then I'd look at loose or damaged connectors, cracked PCBs, things like that.

It's always good to test your test setup - take the antenna off and put a load on instead, and see what happens. Or a short/open. None of them should radiate to any significant degree, so if your readings change with a load on the end of the cable, then something is going on.

I should add, if the physical size of the vna is comparable to the physical size of the antenna under test, the affect of the vna is significant. On the other hand, for example, I measured a 40 meter dipole 10 feet above ground. It was ~ 67 feet in length and flat. An NVIS antenna. The vna was clipped onto the dipole center arm and the affect of holding it is nil. The resonance of the antenna was obvious as well the swr spot on. Since I was standing at the antenna I was in the near field. However, at 6 Foot 7... I am physically small compared to 67 feet!

Alan

Thanks all for your great answers! There was indeed no ground, so stupid me
:) One of the antennas was a groundplane antenna, and I would have
expected somewhat better behaviour there. It was for 2m and I was still
fairly close by. I will try again with the antenna on a small mast about 3m
off ground.


Am Sa., 22. Aug. 2020 um 07:00 Uhr schrieb alan victor <
avictor73@...>: